page_title: Fixation disparity
section_title: Methods for measuring fixation disparity
hierarchical_section_title: Fixation disparity / Methods for measuring fixation disparity
caption_attribution_description: English: Definition of fixation disparity
context_page_description: Fixation disparity is a tendency of the eyes to drift in the direction of the heterophoria. While the heterophoria refers to a fusion-free vergence state, the fixation disparity refers to a small misalignment of the visual axes when both eyes are open in an observer with normal fusion and binocular vision. The misalignment may be vertical, horizontal or both. The misalignment is much smaller than that of strabismus. While strabismus prevents binocular vision, fixation disparity keeps binocular vision, however it may reduce a patient's level of stereopsis. A patient may or may not have fixation disparity and a patient may have a different fixation disparity at distance than near. Observers with a fixation disparity are more likely to report eye strain in demanding visual tasks; therefore, tests of fixation disparity belong to the diagnostic tools used by eye care professionals: remediation includes vision therapy, prism eye glasses, or visual ergonomics at the workplace.
context_section_description: The methods can be explained based on the study of Hofmann and Bielschowsky in 1900, who applied a modified Maddox wing: the right eye is presented with a horizontal scale and the left eye with an arrow. The observer perceives that the arrow points onto one of the numbers on the scale which indicates a possible vergence mis-adjustment. The Maddox wing, however, does not test binocular vision since no fusion target is present. For testing the state of binocular vision, Hofmann and Bielschowsky included an additional fusion stimulus to the two eyes and still found a perceived offset of scale and arrow; they referred to this offset as “Disparitätsrest” (in German), which means “residual disparity”. Later, Ogle coined the term "fixation disparity".
  More generally, this traditional vergence test is a subjective test in the sense that the observer reports his/her perception of the relative position of two test targets that are presented separately to the two eyes, i.e. dichoptic targets. This test relies on the assumption that retinal points are associated with visual directions in space. If physically aligned dichoptic targets appear subjectively aligned, they are projected onto corresponding retinal points and the visual axes intersect at the test target; thus, the vergence angle agrees with the viewing distance. In case of a deviating vergence state, the dichoptic targets need to have a certain physical horizontal offset in order to be perceived in line. These subjective measures agree with objective recordings with eye trackers, if no fusion stimulus is involved.
  For measuring subjective fixation disparity, researchers as Ogle, Sheedy and Saladin, Mallett, Wesson constructed test instrumentation including fusion targets and dichoptic targets using cross-polarized filters in front of the eyes; some of these devices are commercially available. If the dichoptic targets are presented to the observer in physical alignment, the angular amount (in the unit minutes of arc) of subjective fixation disparity is indicated by the perceived misalignment of the two dichoptic targets. This can be compensated by the patient's individual amount of a prism eye glass (in the unit prism dioptre) so that the patient perceives alignment. The latter prism needed  to reduce the fixation disparity to zero is referred to as aligning prism (earlier called associated phoria). Instrumentations as the Disparometer, the Mallett-unit, or the Wesson Card differ in the type of fusion target: some use small central fixation letters, others use more peripheral fusion targets. The instruments can be swung through 90° to measure any vertical fixation disparity. The test devices can also be used to detect suppression.
  The above studies of subjective fixation disparity assumed - partly implicitly - that the dichoptic targets would indicate the vergence misalignment of the visual axes muscles, i.e. the vergence error, as it can be measured with eye tracking methods. This seemed to be justified by the first objective recording of fixation disparity made in 1960 by Hebbard with an eye tracking method based on small mirrors fixed onto contact lenses: he found agreement between the two measures (in the one observer tested). However, subsequent studies found that the objective recordings with eye trackers can differ substantially from the subjective test results with dichoptic targets: with central fusion targets and closely adjacent dichoptic targets, the subjective measure can be about 10 times smaller than the objective measure. When the dichoptic targets are gradually shifted by some degree away from the fusion target, then the two measures become more and more similar. This was interpreted as a change in retinal correspondence in the sense that the visual direction associated with the dichoptic targets is modified in the vicinity of the fusion target.
caption: Fig. 2: Definition of the two types of fixation disparity
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